Spark gap device for lightning arrester

ABSTRACT

Two discal electrodes are eccentrically and rotatably mounted on each of two metallic rods in superposed relationship with each other to form therebetween discharge gaps in parallel relationship having uniform and nonuniform electric fields. Recesses are formed on those portions of arc quenching plates facing the spark gaps.

United StatesPatent U91 Yamada et al.

[ SPARK GAP DEVICE FOR LIGHTNING ARRESTER [75] Inventors: Naoya Yamada; Nobuo Nagai; Shoji Tada, all of Amagasaki, Japan [73] Assignee: Mitsubishi Denki Kabushiki Kaisha,

Tokyo, Japan [22] Filed: Dec. 14, 1970 [21] Appl. No.: 97,999

52 U.s.c|. ..3l5/36,313/D1G.5 51 1m. (:1. ..H02h 7/24 [58] Field ofSearch ..313/DIG.5;315/36 [S6] 5 References Cited UNITED STATES PATENTS 3,489,949 1/1970 Carpenter; 315/36 [451 Mar. 19, 1974 3,414,759 12/1968 Connell et al. 315/36 3,496,409 2/1970 Connell 315/36 3,515,934 6/1970 Kershaw, Jr. 315/36 2.913.626 11/1959 Bislin 313/D1G. 5

Primary Examiner-Nathan Kaufman Attorney, Agent, or Firm-Robert E. Burns; Emmanuel J Lobato 5 7] ABSTRACT Two discal electrodes are eccentrically and rotatably mounted on each of two metallic rods in superposed relationship with each other to form therebetween discharge gaps in parallel relationship having uniform and nonuniform electric fields. Recesses are formed on those portions of arc quenching plates facing the spark gaps. I

9 Claims, 8 Drawing Figures PATENIEDIIAR 19 1914 SPARKOVER VOLTAGE SPARKOVER VOLTAGE (A.C.)

SHEET 1 I]? 2 FIG.

(PR/0R ART I I0 I00 IOOO SPARKOVER TIME IN MICROSECONDS I l l l IOO IOOO SPARKOVER TIME IN MICROSECONDS FIG. 2

SPARK GAP DEVICE FOR LIGHTNING ARRESTER BACKGROUND OF THE INVENTION This invention relates to a spark gap device for a lightning arrester improved in the sparkover characteristic.

With the recent progress in the technology of extrasuperhigh voltage transmission, the lightning arrester has begun to have more chance to sparkover slow wave surge (switching surge) voltages derived from opening or closing of the line than had the system lightniiig arrester which has heretofore been used. Therefore the recent spark gap for a lightning arrester for eittrasuperhigh voltage is required to have a flat sparkover characteristic exhibiting a substantially constant sparkover voltage with respect to various abnormal voltages within a wide frequency range covering from switching surges having a rising time of almost commercial frequency to lightning surges having an abrupt rising time of about 1 us. v The typicalgap device for an arrester is provided therein .with air or nitrogen gas as an arcquenching medium. On the other hand, it.has been known that the use of an electrically negative" gas such as an SF gas which is extremely superior in arc quenching ability to those previously used can realize a spark gap device not only excellent in power follow current interrupting capacity but also improved in the sparkover characteristic withthe polluted surfaceof an insulator and corona characteristic. The spark gap device employing an SF gas or the like, however, has not yet been put in practical use. Th'is is partly because the sparkover characteristic of the gap in an atmosphere of an electrically negaor nitrogen gas.

FIG. -1 illustrates the sparkover characteristics of two over time and that the dispersion'of the sparkover voltage'is very large with respectto the voltage such as a slow wave voltage or a switching surge of which intensity varies very slowly; A curve II in FIG. 1 shows the V-t characteristic across the nonuniform field gap. The

' curve ll shows thatthe. dispersionof the sparkover voltage with respect to the slow wave voltage(switching surge) is small. But in the case of steep front impulse sparkover, the sparkover voltage is very high.

arresters is derived from the fact that the ion chips and the like which 'are useful in the air or nitrogen gas atmosphere in improving the sparkover characteristic are useless in the atmosphere of electrically negative gases.

When the electrically negative gas such as SF gas is employed as an arc quenching medium in the spark gap device, the gap distance itself can be made very short owing to the excellent insulating ability of the electrically negative gas used. Such a spark gap device, however, is disadvantageous in that the sparkover often takes place only along the surface of the arc quenching plate because the distance between the electrodes is shortened. This has resulted in deterioration of sparkover characteristic of the whole spark gap device. In addition, when the sparkover takes place across the spark gap, the deterioration of the surface of the arc quenching plate will result.

Furthermore, if an arc driving coil is disposed close to the arc quenching plate, the electric field established by the vdriving coil is concentrated at one portion of the surface of the arc quenching plate. This causes the surface insulation to easily break down.

SUMMARY OF THE INVENTIQN Accordingly, a chief object of the invention is to prov vide a spark gap device exhibiting a flat V-t characterisnegative gasses such as an SP gas.

Another object of the invention is to provide a spark gap device wherein the are generated across either of the round electrodes having therebetween a uniform field or the sharp electrodes having therebetween a nonuniform field is directly driven to the driving elec- Although the sparkover characteristics as above describe can be observed also in an air or a nitrogen gas atmosphere, they are conspicuous particularly in electrically negative gases such as SF having a powerful aftrodes from the round electrodes without passing through the sharp electrodes or from the sharp electrodes.

Still another object of the invention is to provide a spark gap device wherein surface insulation breakdown can hardly take place at that portion of the arc quenching plates facing the spark gap even when a narrow spark gap is employed owing to the use of an electrically negative gas.

The invention accomplishes these objects by the provision of a spark gap device comprising a uniform field gap having a uniform electric field distribution and a nonuniform field gap having a nonuniform field distribution connected in parallel with each other. The

round electrodes forming the spark gap having the uniform field distribution and the sharp electrodes forming the spark gap having the nonuniform field distribution are disposed in superposed relationship with each other to be mounted on each of common electrode mounting rods separately and rotatably. In addition, recessed portions are formed on that portion of the arc quenching plate facing the spark gaps for the purpose of substantially extending'the surface insulation distances.

' According to the invention there is provided a spark gap device for a lightning arrestercomprising a spark gap having a uniform electric field and a spark gap having a nonuniform electric field connected in parallel with each other, said spark gap having the uniform electric field being arranged to discharge in an impulse voltage region and said spark gap having the nonuniform electric field being arranged to discharge in a slow wave voltage switching surge region.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is graph showing the V-t characteristics of the spark gaps having the uniform field distribution and the nonuniform field distribution;

FIG. 2 is a schematic diagram showing the embodiment of the invention;

FIG. 3 is a graph showing the V-t characteristic of the spark gap device illustrated in FIG. 2;

FIG. 4 is a schematic diagram illustrating another embodiment of the invention;

FIGS. 5 and 6 are schematic diagrams illustrating other embodiments of the invention;

FIG. 7 is an enlarged plan view of the main portion' of the device shown in FIG. 6; and

FIG. 8 is a sectional view taken along the line VIII- -VIII of FIG. 7.

Throughout several Figures the same reference numerals designate the identical or corresponding components.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawing and in particular to FIG. 2 thereof, the spark gap device of the present invention generally designated by the reference numeral 10 comprises a uniform electric field distribution gap 12 and a nonuniform electric field distribution gap 14 connected in parallel. It is recalled that the uniform field distribution gap 12 has the sparkover characteristic as illustrated by the curve I in FIG. 1, while the nonuniform field distribution gap 14 has the sparkover characteristic as illustrated by the curve II in FIG. 1. Therefore, when these two types of gaps are used in combination as shown in FIG. 2, the resultant sparkover characteristic of the spark gap device 10 becomes as shown in FIG. 3. This is because the sparkover.characteristic is determined by the spark gap exhibiting the lower sparkover voltage characteristic for the respective sparkover time. Therefore, it is easily understood that there is the combination of the gap distances preferable for the lightning arrester. This is accomplished by selecting the gap distances of the uniform and nonuniform gaps l2 and 14 so that the sparkover due to the slow wave voltage or the switching surge, the magnitude of which varies very slowly, always takes place across the nonuniform field distribution gap 14, and that the sparkover due to the lightning surge voltage always takes place across the uniform field distribution gap 12. In other words, the spark gap 12 discharges when the voltage in an impulse voltage region is applied, while the spark gap 14 discharges when the voltage in a slow wave voltage region is applied. The spark gap device 10 having a combination of the gap distances thus determined exhibits a V-t characteristic as illustrated in FIG. 3. It is seen from the characteristic curve that the sparkover voltage is substantially constant throughout various sparkover times and that the dispersion of a.c. voltage is restrained to be very small. In other words, there is provided a sparkover characteristic preferable for lightning arresters.

Referring now to FIG. 4 wherein a combination of the spark gap device 10 shown in FIG. 2 and the conventional spark gap device of the auxiliary gap type is illustrated, it is seen that the spark gap device 10 of the invention comprising the uniform and nonuniform gaps l2 and 14 respectively is used as an auxiliary gap device. The auxiliary gap device 10 is connected through an impedance 16 to a junction 18 formed between two main gaps 20 and 22 shunted by shunting elements 24 and 26 respectively. As is well known, as understood from the V-t curves of the main gaps 20 and 22 and the auxiliary gaps 12 and 14, the sparkover voltage due to the lightning surge voltage always exhibits a higher value than that due to the slow wave voltage, and if an abnormal voltage is applied across terminals 28 and 30, the auxiliary gaps 12 and 14 discharge at first to apply a massive overvoltage across the main gap 20 leading to a discharge of the gap 20 without any delay. Therefore, the main gap 22 discharges in succession to the discharge of the gap 20.

Thus,'the resultant sparkover characteristic of the whole device depends upon the sparkover characteristic of the auxiliary gap device 10. Consequently, when the spark gap device employs a combined gap such as the spark gap device 10 illustrated in FIG. 2 as an auxiliary gap, there is provided a spark gap device having a desirable sparkover characteristic as shown in FIG. 3 because of the same reason as in the case of the gap device 10 shown in FIG. 2.

The invention has been described in terms of a spark gap device wherein the single spark gap is replaced by the combined spark gap having uniform and nonuniform field distribution gaps in the electrically negative gases such as an SP gas in order to improve the sparkover characteristic. It is, however, to be understood that the invention is also very useful for obtaining a spark gap device exhibiting a low impact ratio and a dispersion low in sparkover voltage because, even in an atmosphere of air or nitrogen gas, the sparkover characteristic becomes that shown in-FIG. 3 when the pressure is higher than the atmospheric pressure. FIG. 5 illustrates another embodiment of the invention concerning the disposition of the gap electrodes. In FIG. 5, it is seen that the spark gap deivce 10 for lightning arresters comprises an arc quenching plate 32 provided thereon with driving coils 34 for magnetically driving the generated arc. On the surface of the arc quenching plate 32, there are disposed two pairs of are generating electrodes 36 and 38 defining therebetween a uniform field gap 12 and a nonuniform field gap 14 respectively, and a pair of driving electrodes 40. A characteristic element 42 is connected in series to the gaps 12 and 14.

The spark gaps l2 and 14 forming therebetween a uniform and a nonuniform electric field respectively has a similar sparkover characteristic to those illustrated in FIG. 2. More specifically, the spark gap 12 has a sparkover characteristic as shown by the curve I in FIG. 1 wherein the sparkover characteristic is flat and the dispersion of the discharge is small within the impulse sparkover region, while the spark gap 14 has a sparkover characteristic as shown by the curve II in FIG. 1 wherein the sparkover characteristic is extremely high at the left side or high in impulse ratio and small in discharge dispersion within the slow wave voltage sparkover region(or switching surge voltage region). Consequently, according to the lightning arrester shown in FIG. 5, the impulse sparkover takes place across the gap electrodes 36 while the slow wave voltage sparkover takes place across the electrode 38. This enables the sparkover characteristic to be flat and small'in dispersion as shown in FIG. 3. The spark gap device shown in FIG. 5, however, is disadvantageous in that it requiresa certain period of. time to extend the arc across the gap electrodes 36 by the driving electrodes 40 having a wide gap therebetween. This is because the arc generated across the gap electrodes 36 is at first transferred to the sparking electrodes 38 and, in turn, to the driving electrodes 40, while the arc generated across the sparking electrodes 38 is immediately transferred to the driving electrodes 40. FIGS. 6 to 8 inclusive illustrate the spark gap device effected improvements on the spark gap device illustrated in FIG. 5.

In these Figures, it is seen that the spark gap device 10 of the invention comprises an arc quenching plate '32 having disposed on its surface a pair of arc generating electrodes 36 forming therebetween a uniform field distribution gap 12 and a pair of arc generating electrodes 38 forming therebetween a nonuniform field distribution gap 14 in superposed relationship with each other. The electrodes 36 and 38 are in the form of discs having eccentric through holes 70 and 72 respectively for the purpose which will becomeapparent later. In

order to electrically connect both the electrodes 36 and '38 to each other. and to a driving'coil 34, a lead plate 42 is interposed between the driving electrodes 40, the are generating electrodes 36 and the arc quenching plate 32. It is to be. noted that the coil 34 is adapted to exhibit an electric potential different from that'of at least one of the electrodes 36 and 38m to exhibit an electric potential substantially equal to that of metallic electrode holding members 54. As is apparent from FIGS. 7 and 8, the direction in which the are generating electrodes 36 and 38 oppose the driving electrodes 40 is perpendicular tothe direction in which the arc generatingelectrodes 36 and 38 are aligned.

In order to prevent the generated are from passing through the lead plate 42, there is formed a recessed portion 44 on the lead plate 42 (FIGS. 6 and 7) at that portion thereof between both the arc generating electrodes 36 and 38 and the driving electrode 40 to oppose each other. i

' InFIG. 8, it is seen that the spark gap device comprises an arc quenching plate 46 disposed opposingly to the arc quenching plate 32 to sandwich therebetween the are generating electrodes 36 and 38. The are quenching plates 32 and 46 are provided on their opposing surfaces with two pairs of recesses 48 and50 respectively. Each of the recesses 48 formed on the arc quenching plate 32 is filled with a filling metal 52 to hold therein an electrodeholding rod 54. The recesses 50 formed on the arc quen chingplate 46 serve to accept and hold the heads 56 of the rods 54.

It is also seen' that recessed portions 58 and 60 are formed on those portions of the arc quenching plates 32 and 46 facing each other and between the electrodes 36 and 38 respectively.

To hold the various components in their predetermined positions, both the end portions of the arc quenching plates 32 and 46 are provided with through holes 62 and 64 respectively through which screws 66 extend to fix both the arc quenching plate 32 and 46. The screw 66 has on its one end a head, and on the other end a screw threaded portion to which a nut 68 is to fit.

Preferably, the arc quenching plates 32 and 46 are made of acetal resin and the electrodes 36 and 38 are made of graphite. The electrode holding screws 54 are made of suitable metal such as brass or iron, and the screws 66 are made of any suitable insulating material high in mechanical strength such as acetal resin.

When a surge voltage is applied to the spark gap device shown in FIGS. 6 to 8 to produce an arc across any one of the arc generating electrodes 36 or 38., the generated arc is transferred to the driving electrodes due to the magnetic field established by the driving coils 34, thereby to expand and quench the arc generated across the electrode 36 or 38.

Comparing the operation of the device in'FIGS. 6 to 8 with that of the device in FIG. 5, it is easily understood that the spark gap device illustrated in FIGS. 6 to 8 has several advantages. For example, with the device shown in these. Figures, because the generated arc is immediately transferred to the driving electrodes 40 to be expanded therebetween, the arc quenching opera tion is rapidly achieved, and because the magnetic field is of a constant strength for either of the are generating electrodes, a quite steady interruption is achieved without any adverse change in sparkover characteristic of the device in either case of the are generated across the round electrodes 36 having a uniform field or the sharp electrodes 38 having a nonuniform field.

The assembling operation of the spark gap device shown in FIGS. 6 to Band the adjusting operation of the gap distances of the uniform and nonuniform electric field distribution gaps l2 and 14 are performed in the following manner:

Referring to FIG. 8, a lead plate 42 is first placed on the arc quenching plate 32, and then, the round electrodes 36 having a uniform field and the sharp electrodes 38 having a nonuniform field in the named order. The electrode holding rods 54 are inserted and screwed into the filling metals 52 so as to loosely extend through the bores 70, 72 and 74 formed on the sharp electrodes having a nonuniform field 38, the round electrodes 36 having a uniform field and the lead plate 52 respectively.

The adjustment of the gap distances is performed at this step'of assembly. More specifically, the round and the sharp electrodes 36 and 38 are rotated independently of one another about the electrode holding rods 54 so thatthe gap distances therebetween provide such a combination that the discharges due to slow wave voltages are always performed across the nonuniform spark gap 14, while the discharges due to impulse voltages take place across the uniform spark gap 12.

After the adjustment of the gap distances has been completed, the electrode holding rods 54 are firmly screwed into the filling metals 52 to fix both the elec trodes 36 and 38. After that, the arc quenching plate 46 is placed on the assemblage so that the recesses formed on the inner surface thereof snugly receive the respective heads 56 of the electrode holding rods 54. In order to fixedlyattach the arc quenching plate 46 to the assemblage, the elongated screws 66 are inserted to the bores 64 and 62 formed on the arc quenching plates 46 and 32 respectively and fastened by the nuts 68.

As easily understood from the foregoing description taken in conjunction with FIGS. 7 and 8, because each of the round and sharp electrodes 36 and 38 having the uniform and nonuniform fields respectively is arranged in the form of a disc, and because the holding rods 54 about which the electrodes are to be turned upon adjusting the gap distances extend eccentrically through the discs, the machining of the electrodes can be a very high degree of accuracy and the adjustment of the gap distances can be achieved very easily just by turning the electrodes which are eccentrically rotatable discs. This leads to an extremely stable sparkover characteristic.

In addition, the driving electrodes 40 themselves can be designed to be in any preferable structure, disposition and shape which are most preferable for extending and quenching the generated arc because the discharge takes place irrespective of the driving electrodes 40.

Further, according to the invention, the arc generating electrodes 36 and 38 are mounted by the metallic member for modulating the electric field (the electrode holding rods 54 are utilized in this embodiment) and the recessed portions 58 and 60 are formed on that portion of the arc quenching plate 32 and 46 facing the spark gaps formed between the electrodes 36 and 38. Therefore, the electric field along the surface of the arc quenching plates is prevented from being partly concentrated owing to the presence of the electrode holding rods 54 which serve as the metallic member for modulating the concentration of the electric field as well as to the presence of the recessed portions on the arc quenching plates.

Therefore, with the arrangement as above described, the breakdown voltage of the surface of the arc quenching plate can be increased without effecting any change in the shape or without increasing dimensions of the electrodes or the spark gaps. Besides, because the surface distance is substantially increased, there is no fear that the sparkover takes place along the surface of the arc quenching plate even when the arc quenching plates are deteriorated due to the discharge across the gaps.

Although the invention has been described only in terms of a single embodiment of a spark gap device for a lightning arrester, it is to be understood that the invention is also applicable to other electric devices employing spark gaps.

What we claim is:

l. A spark gap device for a lightning arrester comprising: a pair of arc quenching plates, a pair of spaced apart metal fillings disposed on one side of one arc quenching plate at a predetermined distance from each other, a pair of metallic electrode holding members attached to said filling metals, means defining a pair of recesses on one side of the other are quenching plate each receiving one end portion of one of said metallic electrode holding members, means defining a first spark gap comprising a pair of spaced apart electrodes and having a uniform electric field characteristic and meanseccentrically mounting each electrode on one of said metallic electrode holding members for rotation thereon, means defining a second spark gap comprising a pair of electrodes and having a nonuniform electric field characteristic and means eccentrically mounting each electrode on one of said metallic electrode holding members for rotation thereon and superposed on said first mentioned electrodes, a first are driving coil disposed on the opposite side of said one are quenching plate having an electric potential substantially equal to that of one of said metallic electrode holding members, a second are driving coil disposed on the opposite side of said other arc quenching plate having an electric potential substantially equal to that of the other of said metallic electrode holding members, and means defining recessed portions formed on a portion of each of said are quenching plates disposed betwee and spaced from said metallic electrode holding members, and means for pressing together said pair of arc quenching plates and the two pair of superposed electrodes therebetween thereby preventing rotational movement of said electrodes.

2. A spark gap device for a lightning arrester comprising: means defining a first spark gap receptive thereacross during use of the device of an input voltage having a risetime and said first spark gap having a sparkover voltage-risetime characteristic wherein the sparkover voltage is equal to a predetermined substantially constant value for input voltage risetimes greater than a predetermined value and a value no less than said predetermined substantially constant value for risetimes less than said predetermined value; means defining a second spark gap receptive thereacross during use of the device of an input voltage having a risetime and connected in parallel with said first spark gap and having another sparkover voltage-risetime characteristic wherein the sparkover voltage is equal to said predetermined substantially constant value for input voltage risetimes less than said predetermined value and no less than said predetermined substantially constant value for risetimes greater than said predetermined value, whereby the first spark-gap always sparks over first for input voltage risetimes greater than said predetermined value and the second spark-gap always sparks over first for input voltage risetimes less than said predetermined value and thereby said parallel combination of said first spark gap and said second spark gap imparts to the spark gap device a substantially constant sparkover voltage-risetime characteristic.

3. A spark gap device according to claim 2, wherein said means defining said first spark gap includes a first pair of electrodes having a geometric shape effective to impart said sparkover voltage-risetime characteristic to said first spark gap and said means defining said second spark gap includes a second pair of electrodes having a geometric shape effective to impart said another sparkover voltage-risetime characteristic to said second spark gap.

4. A spark gap device for a lightning arrestor comprising: means defining a plurality of main spark gaps connected in series each receptive thereacross during use of the device of an input voltage having a risetime, each spark gap having a sparkover voltage-risetime characteristic wherein the sparkover voltage is greater than a first predetermined value over the range of operative input voltage risetimes; means defining at least one auxiliary spark gap receptive thereacross during use of the device of an input voltage having a risetime and each connected in parallel with one of said plurality of main spark gaps and each comprising means defining a first spark gap having a sparkover voltage-risetime characteristic wherein the sparkover voltage is equal to a predetermined substantially constant value less than said first predetermined value for input voltage risetimes greater than a second predetermined value and a value no less than said predetermined substantially constant value for risetimes less than said second predetermined value and means defining a second spark gap connected in parallel with said first spark gap and having another voltage-risetime characteristic wherein the sparkover voltage is equal to said predetermined substantially constant value for input voltage risetimes less than said second predetermined value and a value no less than said predetermined substantially constant value for risetimes greater than said second predetermined value; whereby the first spark-gap always sparks over first for input voltage risetimes greater than said second predetermined value and the second spark-gap always sparks over first for input voltage risetimes less than said second predetermined value and thereby the parallel combination of said first and second spark gaps imparts to said auxiliary spark gap a substantially constant sparkover voltage-risetime characteristic and the parallel combination of said auxiliary spark gap and said main spark gap imparts to the spark gap device substantially constant sparkover voltage-risetime characteristic.

5. A spark gap device for a lightning arrester comprising: means defining a main spark gap receptive thereacross during use of the device of an input voltage having a risetime and having a sparkover voltage-risetime characteristic wherein the sparkover voltage is greater than a first predetermined value over the range of operative inputvoltage risetimes; means defining an auxiliary spark gap receptive thereacross during use of an input voltage having a risetime and connected in parallel with said main spark gap andcomprising means defining a first spark gap having -a sparkover voltage-risetime characteristic whereinthe sparkover voltage is equal to a predetermined substantially constant value less than said first predetermined value for input voltage risetimes greater than a second predetermined value and I having another sparkover voltage-risetime characteristic wherein the sparkover voltage is equal to said predetermined substantially constant value less than said first predetermined value for input voltage-risetimes less than said second predetermined value and a value no less thansaid predetermined substantially constant value for risetimes greater than said second predetermined value; whereby the first spark-gap always sparks over first for input voltage risetimes greater than said second predetermined value and the second spark-gap always sparks over first for input voltage risetimes less than said second predetermined value and thereby the parallel combination of said first and second spark gaps imparts to said auxiliary spark gap a substantially constant sparkover voltage-risetime characteristic and the parallel combinationof said auxiliary spark gap and said main spark gap thereby imparts to the spark gap device a substantially constant sparkover voltagerisetime characteristic.

6. A spark gap device for a lightning arrester comprising:

means defining a main spark gap including one pair of spaced-apart electrodes and receptive thereacross during use of the device of an input voltage having a risetime and having a sparkover voltagerisetime characteristic wherein the sparkover voltage is greater than a first predetermined value over the range of operative input voltage risetimes;.

' means defining an auxiliary spark gap receptive thereacross during use of an input voltage having Ml a risetime and connected in parallel with said main spark gap and comprising means defining a first spark gap comprising a first pair of spaced-apart electrodes and having a sparkover voltage-risetime characteristic wherein the sparkover voltage is equal to a predetermined substantially constant value less than said first predetermined value for input voltage risetimes greater than a second predetermined value and a value no less than said predetermined substantially constant value for risetimes less than said second predetermined value and means defining a second spark gap connected in parallel with said first spark gap and comprising a second pair of spaced-apart electrodes and having another sparkover voltage-risetime characteristic wherein the sparkover voltage is equal to said predetermined substantially constant value less than said first predetermined value for input voltage-risetimes less than said second predetermined value and a value no less than said predetermined substantially constant value for risetimes greater than said second predetermined value; and mounting means for mounting said first pair of electrodes and said second pair of electrodes in superposed abutting relationship and including means for mounting each pair of superposed electrodes for independent adjustment of said first and second spark gaps; whereby the parallel combination of said first and second spark gaps imparts to said auxiliary spark gap a substantially constant sparkover voltage-risetime characteristic and the parallel combination of said auxiliary spark gap and said main spark gap thereby imparts to the spark gap device a substantially constant sparkover voltagerisetime characteristic. 7. A spark gap device according to claim 6; wherein each of said first pair of electrodes comprise a disc having two main sides and a circumferential side having a bluntly arcuate cross-section and each of said second pair of electrodes comprise a disc having two main sides and a circumferential side having a pointed crosssection.

8. A spark gap device according to claim 7; wherein said mounting means comprises two are quenching members for mounting each pair of superposed electrodes and said one pair of electrodes therebetween each of said arc quenching members having two main sides opposite one another, the inward side of each having means therein defining a recess disposed at a portion of said inward side spaced from and between each of said two pairs of superposed electrodes for increasing the lateral surface distance along the surface of each of said inward sides between each of said two pairs of superposed electrodes.

9. A spark gap device according to claim 8; wherein said means for mounting said first and second pair of electrodes in superposed abutting relationship comprises two electrically conductive electrode holding members, means for mounting said electrode holding members between said two inward sides of said are quenching members, and means for eccentrically mounting each of said two pairs of superposed electrodes on one of said holding members for rotation therein and means for pressing together said are quenching members and said two pairs of superposed electrodes and said one pair of electrodes therebetween and thereby preventing said two pair of superposed electrodes from rotating.

I i l 

1. A spark gap device for a lightning arrester comprising: a pair of arc quenching plates, a pair of spaced apart metal fillings disposed on one side of one arc quenching plate at a predetermined distance from each other, a pair of metallic electrode holding members attached to said filling metals, means defining a pair of recesses on one sidE of the other arc quenching plate each receiving one end portion of one of said metallic electrode holding members, means defining a first spark gap comprising a pair of spaced apart electrodes and having a uniform electric field characteristic and means eccentrically mounting each electrode on one of said metallic electrode holding members for rotation thereon, means defining a second spark gap comprising a pair of electrodes and having a nonuniform electric field characteristic and means eccentrically mounting each electrode on one of said metallic electrode holding members for rotation thereon and superposed on said first mentioned electrodes, a first arc driving coil disposed on the opposite side of said one arc quenching plate having an electric potential substantially equal to that of one of said metallic electrode holding members, a second arc driving coil disposed on the opposite side of said other arc quenching plate having an electric potential substantially equal to that of the other of said metallic electrode holding members, and means defining recessed portions formed on a portion of each of said arc quenching plates disposed betwee and spaced from said metallic electrode holding members, and means for pressing together said pair of arc quenching plates and the two pair of superposed electrodes therebetween thereby preventing rotational movement of said electrodes.
 2. A spark gap device for a lightning arrester comprising: means defining a first spark gap receptive thereacross during use of the device of an input voltage having a risetime and said first spark gap having a sparkover voltage-risetime characteristic wherein the sparkover voltage is equal to a predetermined substantially constant value for input voltage risetimes greater than a predetermined value and a value no less than said predetermined substantially constant value for risetimes less than said predetermined value; means defining a second spark gap receptive thereacross during use of the device of an input voltage having a risetime and connected in parallel with said first spark gap and having another sparkover voltage-risetime characteristic wherein the sparkover voltage is equal to said predetermined substantially constant value for input voltage risetimes less than said predetermined value and no less than said predetermined substantially constant value for risetimes greater than said predetermined value, whereby the first spark-gap always sparks over first for input voltage risetimes greater than said predetermined value and the second spark-gap always sparks over first for input voltage risetimes less than said predetermined value and thereby said parallel combination of said first spark gap and said second spark gap imparts to the spark gap device a substantially constant sparkover voltage-risetime characteristic.
 3. A spark gap device according to claim 2, wherein said means defining said first spark gap includes a first pair of electrodes having a geometric shape effective to impart said sparkover voltage-risetime characteristic to said first spark gap and said means defining said second spark gap includes a second pair of electrodes having a geometric shape effective to impart said another sparkover voltage-risetime characteristic to said second spark gap.
 4. A spark gap device for a lightning arrestor comprising: means defining a plurality of main spark gaps connected in series each receptive thereacross during use of the device of an input voltage having a risetime, each spark gap having a sparkover voltage-risetime characteristic wherein the sparkover voltage is greater than a first predetermined value over the range of operative input voltage risetimes; means defining at least one auxiliary spark gap receptive thereacross during use of the device of an input voltage having a risetime and each connected in parallel with one of said plurality of main spark gaps and each comprising means defining a first spark gap having a sparkover voltage-risetime characteriStic wherein the sparkover voltage is equal to a predetermined substantially constant value less than said first predetermined value for input voltage risetimes greater than a second predetermined value and a value no less than said predetermined substantially constant value for risetimes less than said second predetermined value and means defining a second spark gap connected in parallel with said first spark gap and having another voltage-risetime characteristic wherein the sparkover voltage is equal to said predetermined substantially constant value for input voltage risetimes less than said second predetermined value and a value no less than said predetermined substantially constant value for risetimes greater than said second predetermined value; whereby the first spark-gap always sparks over first for input voltage risetimes greater than said second predetermined value and the second spark-gap always sparks over first for input voltage risetimes less than said second predetermined value and thereby the parallel combination of said first and second spark gaps imparts to said auxiliary spark gap a substantially constant sparkover voltage-risetime characteristic and the parallel combination of said auxiliary spark gap and said main spark gap imparts to the spark gap device substantially constant sparkover voltage-risetime characteristic.
 5. A spark gap device for a lightning arrester comprising: means defining a main spark gap receptive thereacross during use of the device of an input voltage having a risetime and having a sparkover voltage-risetime characteristic wherein the sparkover voltage is greater than a first predetermined value over the range of operative input voltage risetimes; means defining an auxiliary spark gap receptive thereacross during use of an input voltage having a risetime and connected in parallel with said main spark gap and comprising means defining a first spark gap having a sparkover voltage-risetime characteristic wherein the sparkover voltage is equal to a predetermined substantially constant value less than said first predetermined value for input voltage risetimes greater than a second predetermined value and a value no less than said predetermined substantially constant value for risetimes less than said second predetermined value and means defining a second spark gap connected in parallel with said first spark gap and having another sparkover voltage-risetime characteristic wherein the sparkover voltage is equal to said predetermined substantially constant value less than said first predetermined value for input voltage-risetimes less than said second predetermined value and a value no less than said predetermined substantially constant value for risetimes greater than said second predetermined value; whereby the first spark-gap always sparks over first for input voltage risetimes greater than said second predetermined value and the second spark-gap always sparks over first for input voltage risetimes less than said second predetermined value and thereby the parallel combination of said first and second spark gaps imparts to said auxiliary spark gap a substantially constant sparkover voltage-risetime characteristic and the parallel combination of said auxiliary spark gap and said main spark gap thereby imparts to the spark gap device a substantially constant sparkover voltage-risetime characteristic.
 6. A spark gap device for a lightning arrester comprising: means defining a main spark gap including one pair of spaced-apart electrodes and receptive thereacross during use of the device of an input voltage having a risetime and having a sparkover voltage-risetime characteristic wherein the sparkover voltage is greater than a first predetermined value over the range of operative input voltage risetimes; means defining an auxiliary spark gap receptive thereacross during use of an input voltage having a risetime and connected in parallel with said main spark gap and comprising means defining a first spark gap comprising a first pair oF spaced-apart electrodes and having a sparkover voltage-risetime characteristic wherein the sparkover voltage is equal to a predetermined substantially constant value less than said first predetermined value for input voltage risetimes greater than a second predetermined value and a value no less than said predetermined substantially constant value for risetimes less than said second predetermined value and means defining a second spark gap connected in parallel with said first spark gap and comprising a second pair of spaced-apart electrodes and having another sparkover voltage-risetime characteristic wherein the sparkover voltage is equal to said predetermined substantially constant value less than said first predetermined value for input voltage-risetimes less than said second predetermined value and a value no less than said predetermined substantially constant value for risetimes greater than said second predetermined value; and mounting means for mounting said first pair of electrodes and said second pair of electrodes in superposed abutting relationship and including means for mounting each pair of superposed electrodes for independent adjustment of said first and second spark gaps; whereby the parallel combination of said first and second spark gaps imparts to said auxiliary spark gap a substantially constant sparkover voltage-risetime characteristic and the parallel combination of said auxiliary spark gap and said main spark gap thereby imparts to the spark gap device a substantially constant sparkover voltage-risetime characteristic.
 7. A spark gap device according to claim 6; wherein each of said first pair of electrodes comprise a disc having two main sides and a circumferential side having a bluntly arcuate cross-section and each of said second pair of electrodes comprise a disc having two main sides and a circumferential side having a pointed cross-section.
 8. A spark gap device according to claim 7; wherein said mounting means comprises two arc quenching members for mounting each pair of superposed electrodes and said one pair of electrodes therebetween each of said arc quenching members having two main sides opposite one another, the inward side of each having means therein defining a recess disposed at a portion of said inward side spaced from and between each of said two pairs of superposed electrodes for increasing the lateral surface distance along the surface of each of said inward sides between each of said two pairs of superposed electrodes.
 9. A spark gap device according to claim 8; wherein said means for mounting said first and second pair of electrodes in superposed abutting relationship comprises two electrically conductive electrode holding members, means for mounting said electrode holding members between said two inward sides of said arc quenching members, and means for eccentrically mounting each of said two pairs of superposed electrodes on one of said holding members for rotation therein and means for pressing together said arc quenching members and said two pairs of superposed electrodes and said one pair of electrodes therebetween and thereby preventing said two pair of superposed electrodes from rotating. 